David Cameron risked the wrath of MPs in his party and made a formal apology on behalf of the government for the "unjustified and unjustifiable" killings documented by Lord Saville's inquiry, welcoming the "closure" it will now hopefully begin to give grieving families.

While acting with alacrity to apologise, Cameron would not be drawn on whether any of those implicated should be prosecuted and signalled that there would never again be "open-ended and costly" inquiries, though he did not make clear how the government would ensure that.

Some on Cameron's own benches served as soldiers in Northern Ireland as younger men but he was unambiguous that he believed the 12 year investigation showed "on balance" British troops fired the first shots during the "tragic events" in Derry in January 1972 without issuing a warning. Cameron and his Northern Ireland secretary, Owen Paterson received copies of the report on Monday night.

The prime minister told a sombre Commons: "The conclusions of this report are absolutely clear. There is no doubt, there is nothing equivocal, there are no ambiguities. What happened on Bloody Sunday was both unjustified and unjustifiable. It was wrong."

"Some members of our armed forces acted wrongly. The Government is ultimately responsible for the conduct of our armed forces and for that, on behalf of the Government – and indeed our country – I am deeply sorry."

These were "shocking conclusions to read and shocking words to have to say", Cameron said. "But you do not defend the British Army by defending the indefensible. There is no point trying to soften or equivocate what is in the report. It is clear from the tribunal's authoritative conclusions that the events of Bloody Sunday were in no way justified."

The prime minister, who was five at the time of the 1972 incident, added: "For someone of my generation, Bloody Sunday and the early 1970s are something we feel we have learned about rather than lived through.

"But what happened should never, ever have happened. The families of those who died should not have had to live with the pain and the hurt of that day and with a lifetime of loss."

But the prime minister was also careful to soothe unhappiness from military quarters, stressing that Bloody Sunday "was not the defining story of the service the British Army gave in Northern Ireland" and praising their "enormous courage and professionalism" during the 38-year military operation between 1969 and 2007.

After Cameron's statement, Mark Durkan, SDLP MP for Foyle, fought back tears as he read the names of the victims in the chamber, saying it was a day "of huge emotion" for his city. He said that the people of Derry did not just live through Bloody Sunday "but also lived with it".